Russian police investigators work the scene of the suicide attack in Volgograd, about 560 miles southeast of Moscow. / Vladimir Pakhomov, AFP/Getty Images

by Larry Copeland and Andria Yu, USA TODAY

by Larry Copeland and Andria Yu, USA TODAY

Two suicide bombings on consecutive days killed at least 31 people in a southern Russia city, highlighting the terror threat Russia faces as it prepares to host the Winter Games in six weeks.

A suicide bomber on a bus early Monday in Volgograd killed at least 14 people and left nearly 30 wounded, Russian officials said, a day after another suicide bombing killed at least 17 at a railway station in the city.

Vladimir Markin, the spokesman for Russia's main investigative agency, said Monday's blast involved a bomb similar to the one used in Sunday's bombing at the city's train station.

"That confirms the investigators' version that the two terror attacks were linked," Markin said in a statement. "They could have been prepared in one place."

No one immediately claimed responsibility for the attacks, but they came several months after Chechen rebel leader Doku Umarov called for attacks against civilian targets in Russia. Umarov, leader of a terrorist group that calls itself the Caucasus Emirate, has called on Muslims to disrupt the Olympics, which will be held in Sochi in February.

"If you are a terrorist group in the Caucasus, the Sochi Olympics are going to be a very inviting target," said Steven Pifer of the Brookings Institution's Arms Control and Non-Proliferation Initiative.

Some Muslim terrorists view the Olympics as a provocation, says Jeffrey Mankoff of the Center for Strategic and International Studies' Russia and Eurasia Program. Sochi was conquered in the 19th century. "They view it as a provocation on territory they consider stolen from Muslims," he said.

The government has deployed tens of thousands of soldiers, police and other security personnel for the Games, and it has introduced some of the most extensive identity checks and security measures seen at an international sports event.

Suicide bombings have rocked Russia for years, but many have been contained to the North Caucasus, the center of an insurgency seeking an Islamist state in the region. Until recently, Volgograd was not a typical target, but the city formerly known as Stalingrad has been struck twice in two months - suggesting militants may be using the transportation hub as a renewed way of showing their reach outside their restive region.

Volgograd, which lies close to volatile Caucasus provinces, is 550 miles south of Moscow and about 400 miles northeast of Sochi, a Black Sea resort flanked by the North Caucasus Mountains.

Through the day Sunday, officials issued conflicting statements on casualties in the railway bombing. They said the suspected bomber was a woman, then reversed themselves and said the attacker could have been a man.

The Interfax news agency quoted unidentified law enforcement agents as saying footage taken by surveillance cameras indicated the bomber was a man. It reported that a torn male finger ringed by a safety pin removed from a hand grenade was found on the site of the explosion.

The railway bomber detonated explosives just beyond the station's main entrance when a police sergeant became suspicious and rushed forward to check ID. The officer was killed.

Markin said the investigation was ongoing. He said another hand grenade, which didn't explode, was found on the explosion site.

Markin said security controls prevented a far greater number of casualties at the station, which was packed with people as several trains were delayed.

The latest bombings followed an explosion Friday in the city of Pyatigorsk in southern Russia, where a car rigged with explosives blew up on a street, killing three.